But the diplomats say there has been no substantial progress since February when the two sides agreed to restart the investigation.
Since then, the U.N. agency has sought information on three issues: experiments with detonators that can be used to set off a nuclear explosion; separate work on high-explosive charges also used in nuclear blasts, and studies on calculating nuclear explosive yields.
The diplomats said that as of Tuesday evening the Iranian regime had only provided information on the detonators, insisting that they were used only for oil exploration.
While such applications are possible, the agency says that its body of interconnected information suggests that they were being tested for nuclear weapons use.
The diplomats said, although two senior IAEA experts pressed the Iranian regime representatives for seven hours on the weekend during a visit to Tehran, no information has been given on the other two issues.
The head of International Atomic Energy Agency, IAEA, was optimistic last month after holding talks in Iran.
The diplomats who made the remarks to the Associated Press in Vienna demanded anonymity because their information is confidential.
The U.S. says that the U.N. nuclear agency must state that it has been able to complete its investigation to its satisfaction as part of any agreement.
The lack of progress reduces chances that the U.S. and five other world powers will meet a November deadline for a deal with the Iranian regime that shrinks its nuclear program in exchange for sanctions relief, the AP report said.